Temozolomide is the international non-proprietary name used to identify 8-carbamoyl-3-methyl-imidazo[5,1-d]-1,2,3,5-tetrazin-4(3H)-one (I):

Temozolomide is an antitumor agent indicated for treating patients with malignant glioma such as cancer, breast cancer, refractory anaplastic astrocytoma, i.e., patients at first relapse who have experienced disease progression in malignant glioma, glioblastoma multiform and anaplastic astrocytoma, on a drug regimen containing a nitrosourea and procarbazine.
Temozolomide preparations are sold on the US market as hard capsules containing 5 mg, 20 mg, 100 mg or 250 mg Temozolomide (marketed as Temodar® by Schering Corporation, Kenilworth, N.J., USA). In other markets it is sold as Temodal®.
Temozolomide is stable at acidic pH (<5), and labile at pH>7, hence can be administered orally. Temozolomide is spontaneously hydrolyzed at physiologic pH to the active species 5-(3-methyltriazen-1-yl)imidazole-4-carboxamide (MTIC) and to the Temozolomide acid metabolite 3-methyl-2,3-dihydro-4-oxoimidazo-[5,1-d]-tetrazine-8-carboxylic acid (TMA).
MTIC is further fragmented to 5-aminoimidazole-4-carboxamide (AIC) and to methyl-diazonium cation, which is a proximal DNA methylating agent. It is proposed in an article by Clark A. S., Deans B., Stevens M. F. G., Tisdale M. J., Wheelhouse R. T., Denny B. J, and Hartley J. A. titled “Antitumor imidazotetrazines. 32. Synthesis of novel imidazotetrazinones and related bicyclic heterocycles to probe the mode of action of the antitumor drug Temozolomide”, published in J. Med. Chem. 38, 1493-1504 (1995), that a sequence of guanine residues represents an accessible nucleophilic and basic microenvironment in DNA, which would facilitate sequence-selective conversion of Temozolomide to MTIC.
The hydrophobic nature of MTIC enables it to penetrate the blood-brain-barrier membrane, therefore the drug is extensively used to treat also malignant brain tumors. The conversion of Temozolomide to MTIC and the further breakdown of MTIC to AIC and a methyl-diazonium cation is irreversible and pH-dependent. In aqueous buffers, Temozolomide is stable at pH<5, but rapidly decomposes to MTIC at pH>7; in contrast, MTIC is stable at alkaline pH, but rapidly breaks down to AIC at pH<7. Temozolomide has an in vitro half-life of 1.9 hours in phosphate buffer at 37° C. and pH 7.4, whereas MTIC in the same solution has a half-life of about 2 minutes, (see for example, Denny B. J., Wheelhouse R. T., Stevens M. F. G., Lincoln L., and Slack J. A. “NMR and molecular modeling investigation of the mechanism of activation of the antitumor drug Temozolomide and its interaction with DNA”, Biochemistry, 1994, 33, 9045-9051).
A small percentage (about 2%) of an administered Temozolomide dose is metabolized to TMA, the carboxylic acid analogue of Temozolomide (Tsang L. L. H., Farmer P. B., Gescher A., Slack J. A., in “Characterization of urinary metabolites of temozolomide in humans and mice and evaluation of their cytotoxicity”, Cancer Chemother. Pharmacol., 26: 429-436, 1990).
Scheme 1 below depicts the proposed metabolism and degradation pathways of Temozolomide. The sign * shows the position of the 14C-labeled carbon atom. Clinical trials that have been carried out with 14C-Temozolomide showed that the drug is converted to MTIC under physiological conditions, by a non-enzymatic chemical degradation process. (S. D Baker et al. in “Absorption, metabolism, and excretion of 14C-Temozolomide following oral administration to patients with advanced cancer”, Clinical Cancer Research, Vol. 5, 309-317, 1999)

The reactivity of anti-tumor imidazotetrazines such as Temozolomide in organic systems is completely different from the reactivity in aqueous media and is dominated by retro-cycloaddition to the isocyanate and diazo precursors, and the chemistry of their breakdown products as reported by Baig G. U. and Stevens M. F. G., in an article titled “Antitumor imidazotetrazines. Part 12. Reactions of mitozolomide and its 3-alkyl congeners with oxygen, nitrogen, halogen and carbon nucleophiles”, published in J. Chem. Soc. Perkin. Trans. 1 (1987), 665-670.
The original synthesis of Temozolomide involves the reaction of 5-diazoimidazole-4-carboxamide with methyl isocyanate, (see Scheme 2 below). While the reaction time in dichloromethane at 25° C. is very long (20 days), highly pure Temozolomide is obtained in high yield, as reported by Stevens M. F. G., Hickman, J. A., Stone, R., Gibson, N. W., Baig, G. U., Lunt, E., and Newton C. G., in an article titled “Antitumor Imidazotetrazines. 1. Synthesis and chemistry of 8-Carbamoyl-3-(2-chloroethyl)imidazo[5,1-d]-1,2,3,5-tetrazin-4(3H)-one, a novel broad-spectrum antitumor agent”, published in J. Med. Chem. 27, 196-201, 1984.

A synthetic method of preparing Temozolomide, similar to the one described by Stevens et al., in J. Med. Chem. 27, 196-201, 1984, is described by Lunt et al. in U.S. Pat. No. 5,260,291.
While in both of these publications it is reported that the melting point of Temozolomide is around 210° C., Lunt also reports that upon heating “effervescence and darkening from 160° C. to 210° C.” is observed. Such a behavior can be attributed to decomposition.
U.S. Pat. No. 6,844,434 describes the preparation of Temozolomide, alkyl analogs and intermediates thereof. The process, which is depicted in Scheme 3 below, comprises reacting 5-amino-1H-imidazole-4-carboxamide hydrochloride (II) with 4-nitrophenyl chloroformate to afford compound (III), which is subsequently reacted with methyl hydrazine to obtain the corresponding compound (IV), which is cyclized to yield Temozolomide.

Another process for preparing Temozolomide is described in U.S. patent application having the Publication No. 2002/0095036 (see Scheme 4 below). In this process, the imine (V) is converted to 2-cyano-N-(1,1-dimethylethyl)-2-[(diphenyl-methylene)amino]-acetamide, which is converted to 2-amino-2-cyano-N-(1,1-dimethyl-ethyl)-acetamide hydrochloride. The latter is reacted with compound (VI) to obtain 5-amino-N4-(1,1-dimethylethyl)-N1-methyl-1H-imidazole-1,4-dicarboxamide, which is converted to 3,4-dihydro-N-(1,1-dimethylethyl)-3-methyl-imidazo-[5,1-d]-1,2,3,5-tetrazine-8-carboxamide (tert-butyl-Temozolomide), which yields Temozolomide under acidic treatment with concentrated sulfuric acid.

Yet another synthesis of Temozolomide is described by Stevens et al. in J. Org. Chem., Vol. 62, No. 21, 7288-7294, 1997, wherein Temozolomide hydrochloride salt is obtained in 65% yield by the hydrolysis of 8-cyano-3-methyl-[3H]-imidazo-[5,1-d]-tetrazin-4-one with hydrochloric acid, as shown in Scheme 5.

The main disadvantage of this process is the low yield in which Temozolomide hydrochloride is obtained (65%). It is assumed that the relatively elevated temperature of 60° C. used in the process increases the content of decomposition products.
The conclusion derived from the clinical trials described in detail hereinabove, is that Temozolomide is highly unstable in non-acidic pH, labile at pH>7 and stable at pH<5. Due to the intrinsic instability of Temozolomide in basic conditions, the conversion of Temozolomide hydrochloride to Temozolomide base in the usual conditions (treatment with a base) is not a viable option. Thus, there is an unmet need in the art for a simple, efficient and convenient method of converting Temozolomide hydrochloride to Temozolomide base
In most of the processes for obtaining Temozolomide described above (excluding the process described by Stevens et al. in J.Org. Chem. Vol. 62, No. 21, 1997), Temozolomide is obtained as a free base, hence no further neutralization step is required.
It is known to those skilled in the art that a method of choice for converting a salt (such as the hydrochloride salt) of an organic active pharmaceutical ingredient to the corresponding free base is by dissolving the salt in a suitable solvent; adding a base (such as sodium hydroxide); and isolating the active pharmaceutical ingredient free base thus obtained, preferably by filtration.
However, this method is not applicable in the case of Temozolomide since this compound is highly unstable at basic conditions of above pH 7 and thus the use of even a milder base such as sodium bicarbonate is undesired. Furthermore, there is still a need in the art for an improved high-yield industrially convenient process for preparing highly pure Temozolomide, using conventional purification techniques, while avoiding using liquid chromatography in the last reaction step.